The overall aim is to define the respective roles of a putative endogenous circannual rhythm and photorefractoriness in regulating annual reprodcutive cycles in the sheep a model seasonal breeder. Specific aims are: (1) To test the hypothesis that there are endogenous circannual rhythms of gonadotropin and prolactin secretion. This will be tested by placing ovary-intact ewes in photochambers, keeping them under constant LD 12:12 or 16:8 photoperiods and monitoring changes in blood levels of progesterone and prolacting for 4 years. (2) To test the hypothesis that there is an endogenous circannual rhythm to the phase-resetting effects of the length of daily photoperiod in altering LH concentrations. This will be tested by exposing each of two groups of ovariectomized, estrogen-treated ewes to alternating 90 day periods of LD 16:8 and LD 8:16 photoperiods for 18 months. The two groups will be exposed to each photoperiod 180 degrees out-of-phase with each other. Temporal changes in blood LH concentrations will be monitored. (3) To determine which neural structures are required for expression of the endogenous circannual rhythms. Ewes will be subjected to either pinealectomy, lesions of the suprachiasmatic nucleaus, or hypothalamic deafferentation plus the sham operations and then kept under a constant LD 12:12 or LD 16:8 photoperiod to determine if the putative endongenous rhythms are disrupted (4) To determine the time period required for the ewe to recover from photorefractoriness. Photorefractory ewes will be subjected to a "recovery" photoperiod for differing durations and then returned to the original photoperiod. Onset cessation of estrous cycles will be monitored. (5) To test the hyposthesis that onset of photorefractoriness is positively correlated to the strength of the stimulatory photoperiod. At the summer solstice, ewes will be shifted abruptly to two short-day photoperiods of differing length. Progesterone will be monitored to determine the relative intervals between photoperiod shift and onset and secondary cessation of cycles.